This is my first guide, I'm not a native english speaker, so feel free to point any mistakes out.

About this build

Let it be clear: I won't be trying to convince you that this build is the better there is. In terms of DPS and endgame viability, there are far better options on this forum. However, I've had fun with it, I have cleared the Hall of Grandmasters, killed Uber Atziri, Chayula and the Shaper with it (not easily, let's be honest), so I thought I'd share it.

The main idea of the build is to profit from Romira's Banquet and a huge cast speed to generate Power Charges really fast, and to hijack this PC generation to generate other charges, thanks to Tulfall and the Hierophant passive Conviction of Power. Cast speed is provided by tree nodes and by the Arcane Surge buff given by Arcane Blessing.
Defensively, you use MoM and Divine Guidance, some dodge (Darkray Vectors and Acrobatics), and you have to try to avoid dangerous situations with a high cast speed Lightning Warp. Facetanking is fine against most map bosses, but is not an option against Atziri, the Shaper or other hard-hitting bosses.

Pros and Cons

Well, I'm basically a pessimist, so let's begin with the cons:

Cons:
- Not the strongest endgame build. Lacks damage, mainly, even with nice gear.
- Uses Tulfall, so you can't use this build as a league starter (you need to wait until people have farmed breaches).
- DPS estimation requires some math (see math section below). Not a real problem, but if you like to know precisely your DPS, well, that's annoying.
- Can't do no regen or elemental reflection maps. Most other mods are fine, though.
- Doorways. Seriously. Doorways are really annoying as they tend to block your projectiles. Doorways killed me more than once. Fuck doorways (not in the literal sense, that would be ridiculous).

Pros:
- [A mathematician's point of view] Fun use of the game's mechanics: strong items and passives interaction.
- You can play it safe, as Frostbolt has a really long range.
- High cast speed (which implies fast Lightning Warp).
- Immune to elemental ailments.
- Endgame: it's fine up to T15 maps. More difficult encounters need you to have some more expensive gear, but is possible if you don't expect the Shaper to die in a few seconds.

Mandatory stats section

- I currently am level 93 (on my way to 94).
- I have 4150 life and 4050 unreserved mana (with 49% of damage taken from mana before life).
- I use 9 Frenzy Charges, 4 Endurance Charges (5 is possible), and a random number between 0 and 2 of Power Charges, with probability described in the math section at the end of this post; although the "basic" version of this build only yields 7 Frenzy Charges.
- I have 48% dodge against attacks, plus the possibility of using a Vaal Grace.
- I have a 400k-ish Path of Building effective DPS (calculations are too complicated for PoB, see math section, but that's a rough estimate), while you can expect a 200k-ish DPS with more modest gear.
- I really hate lists, so I'll stop boring you and start actually describing the build.

Build mechanics

As I already mentionned, the main idea is to make use of these items:

in order to generate frenzy charges, thus gaining cast speed, damage, move speed and dodge (the corruption on the boots isn't needed at first).

So... How does it work ? The point is to generate lots of charges. When your frostbolts hit monsters, Romira's Banquet will give you power charges (if the hit isn't a crit). You then have a chance (thanks to Power of Conviction) to gain endurances charges. When you reach your maximum number of power charges (that is, 3), Tulfall removes them and gives you a frenzy charge. So, given that you don't land too much critical strikes, you will gain endurance and frenzy charges quite rapidly.
From endurance charges, you get physical damage reduction, some resistances (useful on Elemental Weakness maps), and some life when your CwDT setup triggers (thanks to Kingsguard).
From frenzy charges, you gain lots of increased cold damage, move speed, and dodge (from Tulfall, Surgebinders and Darkray Vectors).

But you also want some critical chance: indeed, you want to trigger Elemental overload at least once every 8 seconds!

For this to work smoothly, you need lots of cast speed: the more cast speed you have, the more charges you can gain, and the more probable it is for you to keep the Elemental Overload buff. For more detailed info on cast speed and crit chance, see math section below.

Gem links

Well, obviously, Frostbolt is your main damage source and should be socketed in your chest armour.
My favoured links are the following, although you can try other relevant support gems and CwDT setups:

I favour Lightning golem for I think cast speed is the relevant stat on this build. Frost Bomb lowers enemy cold resistance, which is nice (even if I use Cold Penetration), and Vaal Grace gives a temporary dodge buff that can prove useful. I use Clarity because I need an important mana regen (because of the high cast speed), but you could probably try playing with no auras and Clear Mind.

Gear

Well, most of the essential gear has already been presented.
On the remaining pieces of gear, you should focus on resistances, life, and mana.
For reference, here is what I currently use:

Note that my amulet and boots are corrupted to give +1 Frenzy: that's one of the ameliorations you should look for if you want to push the boundaries of the build.

Body Armour

If you have a nice 5/6-link and don't want to buy or craft a 5/6-link unique, that's fine. I personally like Kingsguard, but it is not a real requirement for the build to work.
In fact, I'm not even using it anymore. For far better DPS, I'm using the Skin of the Lords linked above, which gives +2 levels to Frostbolt (1 from frostbolt itself, 1 from empower). As it (luckily) has the Elemental Overload notable passive, I removed the latter from the tree for better defenses, which compensates a bit for the loss of life due to the removal of Kingsguard.

- Shroud of the Lightless seems an obvious choice (but it is expensive): life, mana, damage and immunity to physical damage (which should happen quite frequently due to the high cast speed).
- I just obtained a (rather bad) Loreweave and haven't had the time to test it, but life/mana/stats/crit chance/max resistances seems interesting.
- Daresso's Defiance certainly has some potential: dodge is always welcomed, and Onslaught seems interesting.
- Otherwise, a good ol' Belly is always a safe choice.

Gloves

The same apply: Surgebinders are good in terms of damage, but if you need more life, resistances or something else, feel free to use something else. A good roll on Repentance can be nice too, but requires way more strength. Some good rare Fingerless Silk Gloves remain a good option.

Boots

I think you can't beat Darkray Vectors:

+1 frenzy gives a nice damage boost, they give in practice +35 move speed (or more, depending on your frenzy charges total), and lots of dodge.
However, you could also use some good rare boots. A +1 Frenzy corruption is nice but not mandatory.

Helmet

There are some nice unique helmets, but you'll probably need a rare one for resistances, life and mana.
Worth mentionning:
- Asenath's Mask for the cast speed,
- Eber's Unification for the extra chaos damage and some mana,
- Mindspiral for mana and a bit of damage.

Amulet

Again, a rare corrupted amulet is probably for the best. However, you could consider:
- Atziri's Foible if you lack mana and mana regen,
- maybe Hinekora's Sight for more dodging,
- The Aylardex if you're crazy for weird item interactions.

Rings

One of your rings is Romira's Banquet, that's compulsory. Don't corrupt it as you need a bit of critical chance.
As for the other one, a good rare with life, cast speed, mana, resistances, or one of these:

Look for cast speed, life, mana, resistances, eventually spell damage or flat added damage to spells. Oh, and you need at least one Frozen Trail, two are recommended.

For reference, here are the jewels I'm currently using :

The Watcher's Eye I'm using makes me reach 49% of damage taken from mana before life, so I tried to have almost as much mana than life. Without it, you probably want a bit more life and a bit less mana.
Notice that the rest of my jewels focus on cast speed and defenses.

While leveling, you should first take the interesting nodes near the Templar's starting point. It should be enough to go through the game while you build the long branches toward the Assassin/Ranger and the Duelist parts of the tree. I'd suggest going for the Assassin part first as you may want to take a few damage nodes at some point.
As for items: try to get your hands on a good wand, or two if you feel the damage is insufficient. Tulborn can be a nice option with the spell crit nodes in the Witch section, but those will have to be removed later on.
Get your Frozen Trails soon, too, as they'll remove the need for Volley or Multiple Projectiles. Eventually take one of the cast speed cluster if needed.
As always, a Tabula Rasa or a right-coloured Skin of the Lords helps a lot with damage, but is quite mana-intensive.
MoM, some life/mana nodes and Clarity should provide a bit of tankiness, so not much worries about survivability. Try to get life and mana on a few items, though.

When you reach level 65 (or a few levels later), you can switch to Tulfall, but you'll have to wait for level 70 to equip Darkray Vectors and Surgebinders, thus making full profit of frenzy charges.

After level 80, take what you need most: jewel sockets are really nice, but maybe you'll need more cast speed, resistances or life/mana first. Luckily, the tree can provides all of those:

(Yes, that's a shitty Paint job. Maybe I'll replace it with something cleaner when I have some time.)

For reference, here is my current skill tree (remember that I don't need Elemental Overload thanks to my body armour, but you probably do).

Ascendancy

- Conviction of Power is your source of Endurance Charges. You need it, although you can take it last if you're playing softcore.
- Divine Guidance also helps you to survive. Take it quickly, as soon as you have a correct mana pool.
- Illuminated Devotion and Arcane Blessing greatly increase your damage, cast speed and mana regen. While you don't have the latter, replace Faster Casting with Arcane Surge in your Frostbolt links.

Bandits

I'm still unsure about that. Killing all bandits gives you 2 skill points that you can use as needed, it is for sure the more versatile option. But Kraytin's reward is really good too, probably more than what 2 points could give. And if you need mana regen, Alira's reward can be an option.
In doubt, I would kill all bandits, but, well, that's up to you.

Well, I think that's all for general build description. Let's move to MATH.

Math!

I will not bury you under dreadful DPS calculation, as they depend quite a lot on your gear. My point here will be to clarify my build choices, mainly about Cast Speed and Critical Chance.
I'll do that in a spoiler, as I don't want to frighten innocent or fragile readers with Markov chains and probability, and summarize it at the end of the post.

Spoiler

So... What exactly happens with your charges ? I'll consider the case of a single boss with no other monsters around for the sake of simplicity. When there are lots of monsters, the build generates charges almost instantly anyway.
I'll also consider, in a first time, that you are in melee range and that your Frostbolt therefore hit the boss as soon as they're cast. That's generally false against hard-hitting bosses, but I'll show that it doesn't change much, except the calculations become awfully annoying to conduct.

I'm going to refer to Power Charges as "PCs". Similarily, "FC" will mean Frenzy Charge.
I'll also denote you crit probability without PCs as p.

Let's take it step by step.
- Starting with 0PC, you can either get a crit, with probability p, or get a non-crit with probability 1-p. As you're using spell echo, the crit roll will be the same for the following cast: if you did get a crit, you stay at 0 PC (Romira's Banquet removes all your PCs). If you didn't, your two casts (the initial one plus the repeated one from Spell Echo) will each give you a PC, so you jump to 2PCs.
- Starting with 1PC, your crit probability is increased by 0.4*0.05 = 0.02. So, with probability p + 0.02, you cast two crit frostbolts, which leads you to the 0PC state (because of Romira's). Otherwise, with proba 0.98-p, you don't crit and therefore gain 2PCs, which leads to a removal caused by Tulfall and the generation of 1FC.
- Starting with 2PCs, your crit proba is p + 0.8*0.05 = p + 0.04. If you crit, you go back to 0PCs because of Romira; if you don't you gain successively 2PCs : the first one leads to a removal (Tulfall !) of all PCs and a FC gain, the second therefore leads to the 1PC state.

That's a bit indigest. If you're lost, use the following diagram :

The numbers on the arrows indicates the probability of going from one state to another, with p your base crit chance for frostbolt.

Red: critical strikes
Orange : non-critical strikes

From that, using a bit of math magic (Markov chains), we can deduce the probability, when casting non-stop (that's usually not the case as you want to blink from time to time, but that's the usual assumption people make when calculating DPS), of having a given number of Power Charges. I'll spare you the actual calculation (mostly, solving a system of three equations); here are the results (as functions of your crit probability with 0 power charges) :

So... As you can expect, the higher your crit probability is, the more probable it is to have 0PC, as Romira's Banquet removes all power charges on a critical hit. From those functions, we can deduce the two data that are important to us : the probability of getting at least a crit within 8 seconds (to sustain Elemental Overload), and the probability of getting at least a Frenzy charge within 1 second (to reach your maximum amount of frenzy charges in a reasonable amount of time, and then sustain them):

Well. You can see that having a crit probability of 5% leads to a rather incertain sustaining of Elemental Overload. On the contrary, a high crit probability leads to a slower generation of frenzy charges. So, I think that p ≈ 0.11 is an acceptable compromise. Those functions also show the importance of a high cast speed : if your cast speed is around 6 casts per second (so, as you're using Spell Echo, 3 crit rolls per second), the frenzy charge generation is noticably clunkier than with a cast speed of 7 casts per second, and you'll run out of Elemental Overload way more often.

But wait. If you remember, I supposed that your Fostbolts hit the enemy instantly. What happens if you're at some distance? It leads to a different Markov chain. The idea is to create more states to represent both your number of power charges and the casts "waiting" to reach the enemy. So, you have to consider 3*2^n different states, where n is half the number of frostbolt casted, but not having reached the enemy. That leads to way more painful calculations, so I only did it in the case where there are 2 frostbolt which haven't reached the target (that is, n=1). And, if the functions differ a bit, they're globally similar :

The red and orange curves are actually so close that they're indistinguishable here.

So, I won't be doing any more calculations, and assume that the build has globally the same behaviour when you're far from the target. I really don't want to solve systems of 3*2^n equations with n higher than 1.

Ah, just a word about DPS calculation : Path of Builnding doesn't permit to set a probability of having a given number of power charges, and these impact DPS as they give flat added cold damage thanks to Tulfall. So, DPS calculation would require a bit of work. I'm to lazy right now to do it, maybe later on.

Well, I think that's enough math for now, so I will end this spoiler.

TL;DR: if you want to both sustain Elemental Overload and gain Frenzy Charges quite rapidly, I suggest a cast speed between 6 and 7 casts per seconds (the higher the better), and a crit probability between 0.11 and 0.13 without power charges. My tree and items provide a crit probability of 11% with a 30% increased crit chance on Romira's Banquet. So, DON'T CORRUPT IT ! And try to reroll the implicit passive until you reach at least 28% increased critical chance. If you're feeling that Elemental Overload doesn't trigger often enough, you can add a few crit nodes from the Witch section of the tree, but it will impair your frenzy charges generation, so be careful.